Glass containers are hollow objects made of glass and, for example, pots, jars, bottles, goblets, etc. The contents of composite packaging may be liquid, solid or powder food products for food use such as fruit juices, yoghourts, prepared dishes, or others. Packaging whose contents must be sterilized at high temperature is commonly made by means of a metal lid screwed onto the glass container. Such packaging is generally difficult to open because the force applied during closing is very great so that the inner surface of the lid presses against the sealing ring of the glass container around its entire edge to obtain completely hermetic closure, i.e. to prevent leaks, in particular during sterilization. Such a solution is also expensive.
Also known is composite packaging made up of a glass container onto which a membrane seal or closer, generally composed of an aluminum film and a polymeric film comprising a layer made of a thermoplastic material, is heat-sealed. The thermoplastic material acts as an adhesive between the closer and the sealing ring of the glass container, the sealing ring of the container corresponding to the peripheral zone of the container bounding its opening and more particularly to the zone on which the heat sealing is carried out. In order to obtain improved adhesion between the glass and the thermoplastic, it is known, for example from patent FR 2 435 439, to apply to the sealing ring of the container a coupling agent with a high affinity for glass and with free bonds to form strong bonds with the thermoplastic, this coupling agent being in fact, for example a silane or silicon oxide. Another treatment of glass known from patents FR 2 723 939 and FR 2 519 956 to improve adhesion between the glass and the thermoplastic involves depositing on the sealing ring a coating such as tin oxide or titanium oxide on which a chromium complex is then deposited, for example Volan™ by DuPont. Patents FR 2 712 583, FR 2 723 939 and EP0620202 also disclose that the chromium complex to can be replaced by a zirconium salt, an aluminum salt or a chromium salt respectively. The adhesion obtained between the thermoplastic and the glass container by treating the sealing ring with these metallic oxides and these metal salts is however not sufficient and does not make it possible to obtain adequate sealing when the packaging is sterilized. The rise in temperature of the packaging contents, and therefore the dilation of gases, during the sterilization stage induces high pressures on the closer and create leaks between the inside and the outside of the packaging.
Patent application FR 2 523 112 discloses composite packaging in which the sealing ring of the glass container is treated by means of titanium oxide or tin oxide and then with silane before receiving a layer of an ethylene/acrylic acid copolymer of the “Surlyn™” type by Dupont, this copolymer being used to seal a closer onto the sealing ring. The use of such a copolymer improves adhesion between the glass container and the closer but is not appropriate when it is desired to sterilize packaging at temperatures higher than 115° C., and more particularly for temperatures higher than 120° C., because the melting point of the “Surlyn™” which ranges between 80 and 95° C. is then lower than the sterilization temperature. Consequently, during the sterilization of such packaging, the thermoplastic melts and no longer provides hermetic sealing, especially as the pressures exerted on the closer during sterilization are great.
The same is true of the use of many thermoplastics containing polyethylene, modified polyethylene, or mixtures containing polyethylene, like those presented in patent EP0620202, because such thermoplastics have melting points equal to or lower than the melting point of the polyethylene which varies between 98 and 115° C. and it is consequently not possible to use them to make packaging designed to be sterilized at temperatures higher than 115° C., and more particularly at temperatures higher than 120° C.
The aim of this invention is to obtain composite packaging having satisfactory hermetic closure.
Another aim of this invention is to obtain composite packaging having hermetic closure than can resist a sterilization stage.